This invention relates to a hydrogen storage alloy electrode used as a negative electrode of an alkaline secondary battery and a method for preparing the hydrogen storage alloy electrode, and more particularly it relates to a paste type hydrogen storage alloy electrode in which a great deal of hydrogen storage alloy is present per unit volume and which permits the preparation of a battery having great capacity, is inexpensive, and can be mass-produced, and a method for preparing the same.
Much attention is now widely paid to a hydrogen battery which mainly consists of various hydrogen storage alloys, and uses a hydrogen storage alloy electrode, as a negative electrode, since the above hydrogen battery has high energy density. In particular, the hydrogen storage alloys of LaNi.sub.5 series, CaNi.sub.5 series and the like are expected as base materials for the above electrode, since they have low hydrogen equilibrium pressure.
In this kind of hydrogen storage alloy electrode, charge and discharge reactions take place as follows: At the time of the charge, hydrogen generated due to the electrolysis of water on the surface of the electrode is stored in the hydrogen storage alloy, and inversely at the time of the discharge, the stored hydrogen is released from that alloy and is then reacted with a hydroxyl ion, whereby the charge/discharge reaction is achieved.
Thus, in order to increase the energy density of the battery, there have been suggested two methods. One method intends to increase the amount of stored hydrogen per unit weight of the hydrogen storage alloy in the electrode, and another method intends to increase the amount of the hydrogen storage alloy which can be contained in the unit volume of the electrode.
As a method for preparing this hydrogen storage alloy electrode, for example, there is known a method which comprises first mixing hydrogen storage alloy powder with electrical conducting powder such as carbon black and then sintering the resulting mixture to a porous material (Japanese Unexamined Patent Publication No. 46827/1983) or a method comprises rolling a mixture obtained by kneading hydrogen storage alloy powder, polytetrafluoroethylene (PTFE) and viscosity increasing agent, followed by contact bonding to a current collector (see Japanese Unexamined Patent Publication No. 66366/1986).
In addition, the following other methods are also known:
(i) A method comprising the steps of kneading hydrogen storage alloy powder with Teflon grains, forming the mixture into a sheet, and pressing the sheet against a net-like current collector; (ii) a method comprising the step of packing a three-dimensional electrode core with a hydrogen storage alloy in the state of powder or a paste of the powder; (iii) a method comprising the steps of mixing a binder with hydrogen storage alloy powder and then pressing the mixture into pellets; (iv) a method comprising the step of rolling a hydrogen storage alloy; and (v) a method comprising the steps of kneading hydrogen storage alloy powder with a polymer binder such as polytetrafluoroethylene (PTFE), polyvinyl alcohol (PVA) or carboxymethyl cellulose (CMC) and an electrical conducting material to form a paste, coating a net-like current collector such as a punched metal with the paste, drying it, and then subjecting the entire current collector to a pressure molding treatment by use of a roller press. The pressure molding treatment in the last method (v) contemplates increasing the amount of the hydrogen storage alloy included in the unit volume of the prepared electrode.
However, each of the above methods for preparing the electrode has some drawbacks. That is, in the method (i), it is difficult to continuously prepare the electrodes. In the method (ii), the three-dimensional core which is a current collector is very expensive, and thus the industrial value of the electrode containing such a core is low. In the method (iii), it is difficult to make a large-scale electrode, enough to make a wound from electrode used in a cylindrical secondary battery. In the method (iv) of rolling of alloy, its use application is limited by a selected kind of alloy, and this method cannot be applied to hard and brittle alloys such as La series hydrogen storage alloys. Moreover, in preparing the hydrgen storage alloy electrode in accordance with the method (v) (a paste system), the following problem is posed:
That is, when a current collector net is coated with such a paste as mentioned above and is then dried, if the selected binder is CMC, the coated paste is solidified after the drying and is then easily peeled down from the current collector net in the subsequent pressure molding process. In such a state, the amount of the hydrogen storage alloy contained in the unit volume of the electrode is not so much as expected.
For the purpose of solving this problem, there have been suggested methods of using, as the current collectors, various kinds of foamed metals having a three-dimensional structure and sintered metallic fibers, but these current collectors all are very expensive, and so their industrial value is low.